Okay, this became a burning question, and I had time to kill, so I did some research and…
‘According to Michael Quinoin at World Wide Words, pants are a pair because, “before the days of modern tailoring, such garments, whether underwear or outerwear, were indeed made in two parts, one for each leg. The pieces were put on each leg separately and then wrapped and tied or belted at the waist (just like cowboys’ chaps). The plural usage persisted out of habit even after the garments had become physically one piece.”‘ http://walkinthewords.blogspot.com/2009/05/phrase-etymology-pair-of-pants.html
Signed,
I-Just-Have-To-Know!